1. Field of the Invention
The present invention relates to techniques for measuring the speed of a vehicle traveling along a railway. More particularly, the invention relates to an apparatus and method for determining the speed of a rail vehicle based on a contactless technique for measuring the angular velocity of the vehicle wheel and axle assembly.
2. Description of the Prior Art
A common technique for measuring the speed of a railway vehicle involves mechanically coupling the shaft of an electrical generator to the end of one of the vehicle's axles. As the axle rotates, the generator produces a proportional output, which may be easily measured or recorded. This generator is commonly referred to in the art as an axle generator.
Typically, the axle generator is mounted to the outside of a journal box covering the otherwise exposed end of the axle. The journal box is an unsprung component in the vehicle suspension. As such, the axle generator and mounting components are subject to the severe shocks and vibrations imposed on the vehicle wheel and axle assembly. Additionally, when used on a diesel-electric locomotive, the axle bears the unsprung portion of the weight of the traction motor. As a result, the shock loads are especially high in this situation. Due to this severe operating environment, it is a common occurrence for bolts holding the axle generator to break or loosen, leading to axle generator failure. Electrical connections to the axle generator will also often become loose or disconnected.
Because the locomotive crew frequently has no other convenient method of speed measurement, failure of an axle generator may create an undesirable operating condition. Additionally, certain locomotives depend upon axle generator output to initiate "transition." The term "transition" refers no the operational changeover in which the electric motors of the locomotive are switched from being connected in series to being connected in parallel. Series connection is generally used at low speeds to provide high current, and thus high torque, to all of the motors. At higher speeds, parallel connection is utilized so that each motor is supplied with sufficient source voltage to exceed the rising counter electromotive force. If the locomotive depends upon the axle generator to initiate transition, it may be rendered inoperable by an axle generator failure.
Some modern locomotives are furnished with radar speed measurement equipment. These radar systems emit an electromagnetic signal which is reflected from stationary objects, such as railway ties, situates along the track route. The reflected radar signal is then received by the locomotive and processed to determine operating speed. While such systems are generally effective, they tend to be relatively expensive. Additionally, radar speed measurement systems are difficult to retrofit onto older locomotives.